Evolution of microsporogenesis in angiosperms

Microsporogenesis is highly labile in early-branching angiosperms, i.e., those with mostly sulcate pollen, compared with the tricolpate and tricolpate-derived eudicots. New records of microsporogenesis in basal angiosperms (19 taxa were examined), together with a review of the literature, demonstrate that the existing typology has been too strictly applied; several basal angiosperms have apparently intermediate forms and therefore do not fit easily into simultaneous or successive categories. Intermediate forms include the "modified simultaneous" type, where ephemeral cell plates are formed after the first meiotic division but then disperse, and simultaneous cleavage follows the second meiotic division. This relative diversity reflects a range of variation in number and position of pollen apertures in basal angiosperms, although both monosulcate and inaperturate pollen may occur in conjunction with either simultaneous or successive microsporogenesis. However, many taxa with inaperturate pollen have successive microsporogenesis, whereas many monosulcate taxa have the simultaneous type (although successive and monosulcate is common in monocotyledons). The predominance of simultaneous microsporogenesis in extant basal angiosperms and in land plants in general (including gymnosperms) indicates that simultaneous microsporogenesis is plesiomorphic in angiosperms, despite the occurrence of the successive type in the putative first-branching extant angiosperm, Amborella. This conclusion contradicts earlier views on the evolutionary polarity of this character.